1. Field of the Invention
This invention relates to the field direct energy conversion. In particular, it relates to the isolation of component cells in monolithically interconnected modules ("MIM").
2. Description of the Related Art
Thermophotovoltaic ("TPV") energy systems convert thermal energy to electric power using the same principle of operation as solar cells. In particular, a heat source radiatively emits photons that are incident on a semiconductor TPV cell. Photons with an energy greater than the bandgap (E.sub.g) of the semiconductor cell excite electrons from the valence band to the conduction band (interband transition). The resultant electron hole pairs (ehp) are then collected by the cell junction. Photo-current/voltage is then available in external metal contacts that can power an electrical load.
The voltage produced across the electrodes of a single TPV cell is, however, insufficient for most applications. To achieve a useful power level from TPV devices, a number of individual photovoltaic cells must be electrically connected in a series/parallel arrangement, which is referred to herein as a photovolatic "module." These modules can be created in a monolithic configuration on a single substrate and, as such, are referred to herein as monolithically interconnected modules ("MIM"). MIMs provide a number of advantages which are useful in the application of TPV systems, including a reduction in joule losses, flexibility in device design and electrical output characteristics, and simplified thermal management and long-wavelength photon recuperation. This later advantage is primarily due to the ease in application of metallic back-surface reflectors ("BSRs") to the substrates. For similar reasons, MIMs have also been used for laser power converters fabricated in Al.sub.x Ga.sub.y In.sub.l-x-y As and Ga.sub.0.47 In.sub.0.53 As epitaxial layers grown on semi-insulating, Fe-doped InP substrates. Other types of high-intensity photovoltaic converters, such as concentrator solar cells, could also potentially benefit from the advantages of MIM technology
TPV MIM development efforts have focused on the implementation of Ga.sub.0.47 In.sub.0.53 As/InP double-heterostructure ("DH") converter structures that are epitaxially grown on semi-insulating, Fe-doped in substrates. In this instance, the optical and electrical properties of the (Fe) InP substrates are key to the TPV MIM design. The low free-carrier density allows effective deployment of BSRs, while the high resistivity results in good electrical isolation of the component cells in the MIM.
Thin-film MIMs are typically manufactured by a deposition and patterning method. One example of a suitable technique for depositing a semiconductor material on a substrate is glow discharge in silane, as described, for example, in U.S. Pat. No. 4,064,521. Electrical isolation of the component photocells is typically accomplished with a trench formed through the semiconductor layers and terminating at the semi-insulating substrate, or, when an conductive substrate is used, at an electrically insulating barrier layer. See, e.g. U.S. Pat. No. 5,266,125. Several patterning techniques are conventionally known for forming the trenches separating adjacent photovoltaic cells, including silkscreening with resist masks, etching with positive of negative photoresists, mechanical scribing, electrical discharge scribing, and laser scribing. One objective in forming the trenches is to make them as shallow as possible because deep trenches and barrier layers add to manufacturing costs and made successful MIM processing more difficult. Moreover, electrically isolating the photovoltaic cells using trenches that terminate at or through the substrate precludes the use of certain high performance binary substrates, such as GaSb, which are difficult to render semi-insulating.